[PATCH] D132232: Update coding standards for constexpr if statements; NFC

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman created this revision.
aaron.ballman added reviewers: dblaikie, rjmccall, echristo, erichkeane.
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We currently suggest that users not use an `else` clause after a `return` statement in a prior `if` branch. e.g.,

  if (foo)
    return 1;
  else // Should remove this else clause
    return 10;

however, this suggestion is incorrect for a constexpr if statement because one of the two branches will be a discarded statement and thus can impact template instantiation behavior. This updates the coding standard to make it clear that it's okay to have a `return` after an `else` in a constexpr if statement.

I think this is an NFC change to the intent of the rule, which is why I've not started an RFC for the changes.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D132232

Files:
  llvm/docs/CodingStandards.rst


Index: llvm/docs/CodingStandards.rst
===================================================================
--- llvm/docs/CodingStandards.rst
+++ llvm/docs/CodingStandards.rst
@@ -1026,6 +1026,21 @@
 The idea is to reduce indentation and the amount of code you have to keep track
 of when reading the code.
 
+Note: this advice does not apply to a ``constexpr if`` statement. The
+substatement of the ``else`` clause may be a discarded statement, so removing
+the ``else`` can cause unexpected template instantiations. Thus, the following
+example is correct:
+
+.. code-block:: c++
+
+  template <typename Ty>
+  static int foo(Ty T) {
+    if constexpr (std::is_integral_v<Ty>)
+      return 10 + T;
+    else
+      return 100;
+  }
+
 Turn Predicate Loops into Predicate Functions
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 


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